1. Field of the Invention
This invention pertains to flow controllers and more particularly to analog flow controllers for varying the circulation rate of a fluid in heat exchange relation with a variable temperature heat source such as, for example, a solar collector.
2. Statement of the Prior Art
The use of solar collectors in space heating and hot water systems is well known as are control systems for regulating the temperature of the circulated fluid, typically water. The control systems, which usually regulate a pump interposed in the fluid circulating circuit, fall into two main categories: on-off controllers and proportional controllers.
On-off regulators employ a temperature sensor disposed to sense the temperature of the water at some point in the system, typically at the output of the collector. The signal from the sensor is sent to a pump controller which deactivates the pump whenever the sensed temperature drops below a reference temperature and activates the pump whenever the sensed temperature exceeds the reference temperature. The reference temperature may be directly dependent on some system parameter, such as the storage water temperature, or may be a fixed value independent of system parameters. Examples of typical on-off regulators may be found in U.S. Pat. Nos. 3,998,207, 4,062,349, 4,019,495, 4,034,738 and 4,063,545.
Regardless of the reference signal selected, on-off controllers result in excessive strain on the pump motor due to constant activation and deactivation thereof. Perhaps more importantly, on-off controllers cannot accurately regulate the temperature of the water leaving the collector due to the time lags inherent in a system which varies the flow rate of the water between zero and some fixed rate. Accordingly, such systems are accompanied by relatively wide fluctuations in the collector water temperature. In most applications, this results in reduced efficiency of the solar heating system. For example, where the solar heating system is used as a hot water heater, it is desirable to keep the temperature at the output of the collector at or near a predetermined value. If the temperature is below the predetermined value, the temperature gradient between the collector water and the storage water may be too low with the result that no heat will be available from the solar heater. On the other hand, if the temperature gradient between the collector water and the storage water is too high, excessive heat is lost to the atmosphere. In either case, the result is reduced efficiency.
Proportional controllers operate on the principle that the flow rate of the collector water through the system is proportional to the temperature of the collector water or some temperature difference, for example, the temperature difference between the water at the output of the collector and the storage water temperature. The proportionality constant is determined by the particular system parameters. Unlike on-off controllers, the signal from the proportional controller is utilized to vary the pump speed and hence the flow rate of the water circulated through the collector. Because this virtually eliminates system cycling, strain on the pump motor is reduced. However, although they are an improvement, proportional controllers, like on-off controllers, cannot maintain a fixed temperature at the collector output. For example, when the collector temperature drops due to a change in ambient conditions, the proportional controller will reduce the pump speed and hence the flow rate in an effort to raise the collector temperature. However, because proportional controllers operate on a pre-set proportional relationship between flow rate and temperature, the new set point established by the proportional controller corresponds to a different temperature than the original temperature at the higher flow rate. It is thus apparent that the collector water temperature in a system regulated by a proportional controller is partially dependent on ambient conditions. Exemplary proportional controllers are described in Hawthorne Industries, Inc.'s Product Information Bulletins for their Model Nos. H-1511-A and H-1510-B.
It is therefore apparent that prior art controllers for regulating the flow rate of fluid through a solar heating system are incapable of maintaining a fixed, predetermined fluid temperature at the output of the collector.